Dropping the sump would give internal access to the 'sealed unit' and allow a visual inspection to be carried out on the shaft concerned. I believe Bristow have been doing this, but I'm not sure about other operators. I am sure it would have been done if it had been a requirement, which it wasn't. So HC is correct about that. Maybe Bristow should re-launch their 225 fleet if they and their passengers are so confident in their own ability to detect these defects?
Whether checking Hums before despatch would have prevented the CHC ditching is debatable, it's far more likely the accelerometer would have been checked and the component would have been put on a close monitor, and that certainly wouldn't have prevented the ditching. I am, of course, speculating about that.
The root problem however remains the shaft in question, and why it is suddenly failing on a regular basis after years of trouble free motoring. I have a feeling there is still much more to come out about that.